Part 1: Star Birth

A World of Dust We are interested in this “interstellar medium” because these dense, interstellar clouds (nebulae) are the birth place of stars The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky.

Bare-Eye Nebula: Orion One example of an interstellar gas cloud (nebula) is visible to the bare eye: the Orion nebula

The Life Cycle of Stars Young stars, still in their birth nebulae Aging supergiant Globules = sites where stars are being born right now!

Shocks Triggering Star Formation Trifid Nebula A shockwave passing through a nebula compresses the gasses. If gravity further compresses the gasses, a star is formed.

Sources of Shock Waves Triggering Star Formation 1) Supernovae (explosions of massive stars) 2) collisions of gas clouds

Sources of Shock Waves Triggering Star Formation c) Spiral arms in galaxies like our Milky Way: Spiral arms are probably rotating shock wave patterns.

Protostars Protostars = pre-birth state of stars: Hydrogen to Helium fusion not yet ignited Still enshrouded in opaque “ cocoons ” of dust => barely visible to normal telescopes, but bright in the infrared.

From Protostars to Stars 3. Hydrogen in the core begins to fuse into Helium 1. Gravity contracts the gasses at the center 2. Angular momentum spins and flattens the disk

Part 2: Main Sequence Stars Main Sequence Stars

Maximum Masses of Main-Sequence Stars  Carinae (Eta Carinae) Maximum mass: ~ 100 solar masses

Minimum Mass of Main-Sequence Stars Minimum mass = 0.08 M sun At masses below 0.08 M sun, gas doesn ’ t get hot enough to ignite fusion.  Brown Dwarfs Gliese 229B

Evolution on the Main Sequence A star will move off the Main Sequence when it runs out of Hydrogen in its core All Main Sequence Stars fuse Hydrogen into Helium in their cores.

Part 3: Life and Death after the Main Sequence

What happens after a star moves off the main sequence is determined solely by its starting mass. Only the most massive stars end as black holes A supernova explosion of a M > 8 M sun star blows away its outer layers, so it ends with less mass.

A: Low mass stars

Stars with less than ~ 0.4 solar masses  Hydrogen and helium remain well mixed throughout the entire star. Mass Red Dwarfs live such long lives that no deaths have been recorded, they will ultimately become black dwarfs. A: Low mass stars

B: Medium mass stars (like our sun)

All stars greater that 0.4 solar masses will expand into a Red Giant when they run out of H in their core. Hydrogen in the core completely converted into Helium: Hydrogen burning continues in a shell around the core. Expansion and cooling of the outer layers of the star  Red Giant

Red Giant Evolution He-core gets denser and hotter until the next stage of nuclear burning can begin in the core: Helium fusion into Carbon and Oxygen

The Remnants of Sun-Like Stars: White Dwarfs As more C and O are dumped on to the core, the core collapses and the matter stops reacting.  Formation of a White Dwarf Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!!

The Final Breaths of Sun-Like Stars: Planetary Nebulae The Helix Nebula The outer shell of the star floats off and forms a planetary nebula Have nothing to do with planets! 

The Fate of Our Sun and the End of Earth Sun will expand to a Red giant in ~ 5 billion years Expands to ~ Earth ’ s radius Earth will then be incinerated! Sun may form a planetary nebula (but uncertain) Sun ’ s C,O core will become a white dwarf

C: Massive Stars

The Deaths of Massive Stars: Supernovae Final stages of fusion in high-mass stars (> 8 M sun ), leading to the formation of an iron core Iron core ultimately collapses, triggering an explosion that destroys the star: A Supernova

The Famous Supernova of 1987: SN 1987A BeforeAt maximum

Formation of Neutron Stars Piece of neutron star matter the size of a sugar cube has a mass of ~ 100 million tons!!! A supernova explosion of a star > 8 M sun blows away its outer layers. The central core will collapse and the pressure gets so high that protons and electrons combine to form – NEUTRONS!  Neutron Star

Black Holes Neutron stars can not exist with masses > 3 M sun It will collapse into a single point – a singularity: It Becomes A Black Hole!

Black Holes No object can travel faster than the speed of light => nothing (not even light) can escape from inside the black hole